The question of how large structures in the universe formed, such as galaxies, clusters or black holes, is one of the oldest in cosmology. However, starting in the 1980s, cosmologists noticed a key element in this process: quantum fluctuations.
These are energy changes at specific points in space-time that, according to the inflation model, were decisive in forming what would become the pieces of matter in our universe. Cosmic inflation is a very popular proposal to explain the rapid expansion of the universe in its early moments of life.
Until recently, it was believed that the large structures we see in our universe today, such as massive clusters with tens of thousands of galaxies, would necessarily have formed much later than this stage.
Although, a new one published in Physical Review Letters offers an explanation for the existence of large and very old structures, dating back to when the universe was only 200 to 400 million years old, and which do not fit the predictions of the Standard Model of cosmology.
Quantum fluctuations during the inflation stage, moments after the birth of the universe, were the key to the formation of massive and very ancient structures.
The key to their formation lies in quantum fluctuations during the inflation stage, according to the authors. This would explain the existence of observations with a high redshift, objects “that shouldn’t be there”, formed just a few hundred million years after the formation of the universe, such as the El Gordo cluster or the massive galaxies seen by James Webb Space Telescope.
El Gordo, the largest cluster of distant galaxies ever observed with existing telescopes, was discovered in 2014. “This cluster is a very massive object dating to 6.4 billion years after the Big Bang, something that would have formed very soon. and whose existence could not be explained with previous models”, explains Juan García-Bellido, co-author of the study and researcher at the Institute of Theoretical Physics (IFT, joint center of the Autonomous University of Madrid and the CSIC).
Such clusters of several tens of thousands of galaxies could not, in theory, form until much later. “It was surprising to find such large objects so early. Therefore, an explanation had to be found”, says the physicist.
Years ago, the authors – who also include José María Ezquiaga, from the Niels Bohr Institute, and Vicent Vennin, from the University of Paris – already realized that quantum fluctuations during the inflation phase affected this dynamic acceleration of the universe.
Before the new work, it was believed that the statistical distribution of these quantum fluctuations during the period of inflation formed a very concrete graph, known as a Gaussian bell. But the researchers realized that, applying equations that allow us to look a little further, what we actually get is a non-Gaussian distribution, which features a ‘tail’ region, as seen in the image below.
Study charts. / María Ezquiaga et al./Phys. Rev. Lett.
“This indicates that these fluctuations can collapse into large structures, galaxies, clusters or even larger structures. With the previous hypothesis, where we had a Gaussian-type function, these structures would take much longer to form due to gravitational collapse”, explains García-Bellido.
The new result would explain the formation of large and very massive structures much earlier than would be required by gravitational collapse.
That is, with a Gaussian-like function, these structures would take much longer to form, so structures as large and old as El Gordo could not be explained. However, the new result, which provides functions that have such a ‘tail’, would explain the formation of large and very massive structures very early in the age of the universe, much earlier than would be required by gravitational collapse.
Application on James Webb’s Observations
According to the authors, the interesting thing about the new proposal is that, thanks to these non-Gaussian exponential tails, an explanation can be given, for example, for the recent observations of the James Webb Space Telescope.
Since its launch in early 2022, this observatory has been making very interesting detections that can now be explained through this new result.
For example, galaxies with very high redshifts. Red shift or redshift is a concept that refers to the age of astronomical objects. It is defined as an increase in the wavelength of electromagnetic radiation received by a detector compared to the wavelength emitted by the source.
The researchers used computational methods to calculate the function, which modifies classical evolution.
Furthermore, the formation of larger-than-expected objects in the early universe, as explained by this new result, helps alleviate some tensions between the observations and our standard cosmological model.
The researchers used computational methods to calculate the function, which modifies classical evolution. Using the so-called Fokker-Planck equation, which takes into account this dynamic of quantum fluctuations, an elliptical and lognormal function is obtained, not a Gaussian type.
This new graph is what gives the highest collapse probability for primordial black holes, primitive galaxies and for very massive objects like the El Gordo cluster.
“We had to take non-linear information into account and open our minds”, recognizes García-Bellido. “The new result explains the non-Gaussianities of large-scale structures, which we have finally started to measure with galaxy catalogs.” Ultimately, these fluctuations with non-Gaussian tails help us explain the large-scale behavior of the universe.
In the future, the researchers hope to continue to complement the standard cosmological model by taking into account observations from deep sky telescopes, and whose formation can correspond to this dynamic of quantum fluctuations in the inflation stage, as shown by the result.
The new study also allows García-Bellido to make an interesting reflection from the point of view of the history of science: “Decades ago, many years passed between theory and experimental application. For example, General Relativity could not be applied until the 1960s, or the Higgs was discovered nearly half a century after it was theorized,” he explains. “I am lucky to be working in an age where experimental observations and theoretical predictions can go hand in hand, for at least 20 years.”
Reference:
Jose Maria Ezquiaga et al. Massive clusters of galaxies like El Gordo suggest primordial quantum scattering. physical review letters (2023)
